I was in the same conundrum as you a couple years ago, so I'll give you my opinion.
I was lusting after this lens quite a bit, until I smartened up on a few things which helped in my decision to stick with just the 70-200.
-You only gain one extra stop with the 135. Now, if you're doing really low light work such as for sports where you need a high shutter speed, then this will be important. For me, I was mainly doing portraits in daytime, around sunrise/sunset, and also with strobes. Even for portraits, the 70-200 is better because it'll gain you several stops through IS since the subject isn't moving, whereas the 135 only gains you one stop.
-For most clients, they actually prefer to not have their backgrounds completely blown out. It seems photographers have hard ons for really small depth of field portraits, but in reality, most people out there don't like them. Of course, you can stop the 135 down to f16, but the 70-200 you already own can do that too. I've done comparisons before (such as f1.2 vs f8, f2 vs f11, etc) and the majority of clients prefer the look of a smaller aperture with more depth of field. It's nice to show a little of the background/environment. If you blow it out so much that you can't see what is in the background, you might as well have just shot it in studio. When I shot more wide open, I got comments/complaints such as "why are my ears out of focus". Photographers will inevitably say because they want you to focus on the eyes/face. Here's the thing: if you have to have a person's ears out of focus, not anything (aka everything) further away, in order to properly draw focus/attention to a photograph, you really need to take an introductory course in composition.
-The zoom comes in very handy. Often, I may be clicking well with the client, for instance I get her to laugh. If I'm zoomed in at 200, I can zoom out to 70 and get a different shot with more of her body as it naturally poses during the laughter. I can do the opposite too, quickly change to a more telephoto focal length. This can be done with cropping, but I'd rather keep all the pixels I can.
-Lack of IS on the 135. I know I already mentioned it, but it can help so much for portraits when the light starts to fall at the end of the day since our subjects are stationary.
-That "pop" that people talk about is nothing more than a combination of contrast/microcontrast and an in focus subject vs out of focus background. The 70-200 can do the latter easily (200 at f2.8 provides less depth of field than 135 at f2), and the former can be applied in post processing. Ask any photographer who's well versed in the topics of lenses/light/optics, and you'll find out the reason for "pop" is as listed above. It reminds me of those people who believe homeopathic medicine is something other than expensive water, that it has the "essence" or "memory" of the medication even though it's been diluted so much there's no chance of even a single molecule being in the solution.
Anyway, those ended up being my reasons for sticking with the 70-200. Hope it helps! Good luck!